1. Field of the Invention
This invention resides, broadly, in the field of thermal energy transfer systems, and more particularly relates to a heat pipe which is capable of efficiently transferring heat downwardly. Heat pipes, as a class of thermal energy transfer devices, comprise closed and sealed chambers containing a liquid that absorbs heat from an input source. The liquid is vaporized by the input heat, creating pressure, and these vapors will move to a cooler and therefore lower pressure portion of the chamber where they give up their latent heat of vaporization and recondenses into a liquid. The liquid condensate is returned to the heat-input region where it is re-vaporized to continue the cycle. The continuous migration of the vapor and fluid results in transfer of heat from the input to the output ends of the heat pipe.
2. Description of the Prior Art
Heretofore, most heat pipes utilized capillary action of a wicking material, lining the interior of the heat pipe, to return the liquid condensate to the vaporization region. While such an arrangement is several orders of magnitude more effective than thermal conduction in a metallic solid, it suffers considerable loss by reason of the thermal insulating qualities of the wicking material. Furthermore, the capillary action is not as efficient in transferring the condensate upwardly as it is in transferring downwardly. It is well known to those versed in the art that the greatest loss in heat pipe efficiency occurs when transferring heat downwardly because of the difficulty of returning the condensate upwardly by capillarity.
Typical heat pipes of the prior art are disclosed in U.S. Pat. Nos. 2,350,341 and 2,350,348 to R. S. Gaugler, and 3,606,005 to R. D. Moore, Jr. The latter patent discloses a heat pipe assembly comprising an end-to-end series of short heat pipes, requiring that the liquid in each pipe section need be lifted by capillarity only a relatively short distance. Moore states that an arrangement of ten segments "has from 10 to 100 times the maximum heat flux capacity of an unsegmented heat pipe of the same cross section and length." Nevertheless, this arrangement still suffers the loss of effectiveness attributable to the thermal insulating qualities of wicking material and the slow rate of liquid transfer that is characteristic of capillary devices. Also, the effects of attitude greatly influence the effectiveness of all known prior heat pipes. To overcome the insulating qualities of most wicking materials, some attempts have been made to use metallic wicks. However, such structure still has the serious shortcoming of very slow liquid transfer rate and practical limitations as to the distance of liquid transfer.
An exemplary commercial heat pipe of the wick-return type having a length of 15 centimeters, is rated to have an upward heat transfer capacity of 6,000 watts and a downward heat transfer capacity of 500 watts. Thus, only 8.3% as much heat can be transferred downwardly as can be transferred upwardly by such a device.
There exists many important applications requiring thermal energy transfer in a downward direction, thus precluding the effective use of prior heat pipe devices. For example, the collection of solar heat on the roof of a building requires the transfer of that heat downward to storage or utilization areas.